<；P>；PROBLEM TO BE SOLVED: To provide a method for efficiently reducing blue-green algae at a low cost without applying load to the environment. <；P>；SOLUTION: Duckweed having a rhizosphere microorganism with algicidal properties is grown in the presence of blue-green algae. In this case, it is preferred to grow the total X<；SB>；1<；/SB>；(pieces) of duckweed which satisfies the relationship of formula (2): X<；SB>；1<；/SB>；&ge；Y/(3.6×；10<；SP>；7<；/SP>；) relative to the total Y (pieces) of blue-green algae. Further, in the duckweed, it is preferred to increase the expression level of a gene encoding chalcone synthase. <；P>；COPYRIGHT: (C)2011,JPO&INPIT
